Q.Describe the usefulness of water in biosphere and biological systems.

Water is indispensable to the biosphere and to biological systems in multiple interconnected ways:

• Universal biological solvent: its high polarity lets it dissolve a huge range of nutrients, salts, gases and metabolites, enabling their transport within organisms — e.g., as blood plasma in animals or sap in plants.
• Reaction medium and direct reactant: most biochemical reactions occur in an aqueous medium; water is also a direct reactant, e.g., in photosynthesis ($6CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2$) and in the hydrolysis of food macromolecules (proteins, starches, fats) during digestion.
• Temperature regulation: water's high specific heat capacity and high latent heat of vaporisation allow organisms to buffer against rapid temperature changes and cool themselves efficiently (e.g., sweating, transpiration).
• Structural role: water maintains the turgidity (rigidity) of plant cells via osmotic pressure, and forms a major fraction of the mass of living cells and tissues. …

List water's biological roles systematically: solvent/transport medium, direct reactant in metabolism, temperature buffer, structural/turgidity role, and ha …

• Giving only a single role (e.g., "it's a solvent") without covering the breadth expected — the question asks about usefulness in both biosphere (ecosystem) and biological systems (organism-level). …